Application Note

Design of an integrator
for the Rogowski coil

The Rogowski coil supplies a voltage in proportion to the derivative of the primary current at its terminals.
An electrical integrator circuit is therefore necessary to convert this signal into a signal that is proportional
to the value of the primary current.
The integrator is an essential component in current measurement with the Rogowski coil, and the way thisamplification stage is implemented will have a major impact on the sensor’s electrical performance (linearity,phase-shift and frequency bandwidth). A list of the various critical aspects of such an integrator, with somepossible solutions, is given below:• Very low signal level (for example 20 mV / kA for sensors in LEM’s RT range)
The use of very low noise OpAmps is recommended to optimize the signal/noise ratio
It is necessary to try to minimize the surface of the PCB or possibly to shield the amplification stage toreduce sensitivity to external fields.
• Low cut-off frequency
When an integrator is connected to a Rogowski coil the two form a high-pass filter. Since it will rejectvery low frequencies it is necessary to define the cut-off frequency in order to optimize performance atthe nominal operating frequencies, while still obtaining the best possible response time.

• Rejection of offset
The main problem of a pure integrator lies in the fact that it will integrate the slightest parasitic offset (e.g.
due to the AmpOp), with the effect that the output will always be unstable and will drift sooner or later tosaturate at the upper or lower level. Consequently it is essential to limit this drift, using a static gain or anactive compensation stage:

Application Note

Design of an integrator
for the Rogowski coil

• Total offset rejection
It is possible to completely eliminate the residual offset by adding a capacitive coupling device between
the integrator and the measuring stage:• Phase-shift
The offset rejection circuits described above will generate several degrees of phase error which poses a major problem
for the measurement of power. In this type of application, it is therefore necessary to add a phase-shift compensation
stage, which generally consists of a low-pass filter. Unfortunately, the correction will not be constant, but will dependon the frequency, meaning it will be necessary to optimize the design to minimize the phase difference at thefundamental frequency, typically 16 2/3, 50, 60 or 400 Hz.

WITHOUT
corrector

WITH
corrector• Calibration: active adjustment of gain
A Rogowski coil requires calibration against a reference signal in order to fine-tune its gain which is never exact byconstruction, due to inevitable imperfections in the manufacturing process. In general engineers use the integrator
stage to which an analogue device, such as a potentiometer, is attached. The most recent digital calibration solutions
are more likely to use a microcontroller or the combination of a microcontroller and a PGA (programmable gainamplifier). In all cases calibration is specific to each individual Rogowski coil which must always use the same circuitwith which it has been calibrated.
• Calibration: passive adjustment of gain
Historically, the Rogowski coil was used simply for measurement of the current effective value (rms) without phase
constraint. Many loops offered factory calibration based on a purely resistive or a resistive/capacitive circuit (RCcircuit). While this method continues to be straightforward and economical, unfortunately it does not lend itself topower measurements due to the strong phase error that it generates, and its possible dependence on the frequency(if an RC circuit is used).

When developing the new Rogowski coils, LEM decided to offer a simple and generic product, keeping in
mind that the integrator technology leads to the best performances and is a well known method. Therefore thetransducers of the RT family are not calibrated in the factory, do not use any additional electronic components
or housings and do not require power supply. Using an integrator specific to the device connected to theRogowski such as energy, power quality or pulse power monitor, is a cost effective and high performancesolution.
LEM SA
8, Chemin des Aulx, CH-1228 Plan-les-Ouates
Tel. +41/22/7 06 11 11, Fax +41/22/7 94 94 78
e-mail: Isa@lem.com; www.lem.com